Protective motor control system



Ofit. 24, 1944. M, HYDE PROTECTIVE MOTOR CONTROL SYSTEM F iled May 15, 1942 1.'0 77/77:? in Seconds K d 0 w Y N 4mm w v A w. C .W M 3 2 4 M 0 f c SJIA ML 0 ggm qsssg w. 4.

Patented Oct. 24, 1944 Merritt A. Hyde, Wilkinsburg, Pa., assignor to Westinghouse Electric & Manufacturin Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application May 15, 1942, Serial No. 443,071

4 Claims. (Cl. 175-294) y invention relates to electric control systems and more particularly to systems of control for controlling the operation of electric motors.

With prior art devices, to minimize the time interval of the stoppage of an electric motor connected to a circuit subject to severe shorttime voltage dips, frequent use is made of a so-v called time-delay undervoltage pushbutton in combination with magnetically, operated motor Another broad objector my invention is the provision of an electric system in which commercial electromagnetic devices, such as relays and contactors, pick-up at lower supply voltages than was possible heretofore with such devices.

Another object of my invention is to minimize the outage time of electric apparatus during a short voltage dip, or temporary voltage failure, of the supply for such electric apparatus.

starters. Other objects and advantages will become more To fully appreciate. the novel contribution I apparent from a study of the following specificamake to the art and to fully understand the obtion and the drawing accompanying the specifijects, the description, and claims of my invencationyin which drawing: I tion, a brief description of some prior art appa- Figure 1 is a diagrammatic showing of one emratus will be of value. By one prior art scheme bodiment of my invention; use is made, for examp1e,'of a simple full-voltage Fig; 2 shows a curve that is helpful in describmagnetic starter consisting of a magnetic coning my contribution to the art, and v tactor for making and breaking the connection Figs. 3 and 4; are diagrammatic showings of of the motor to the source of supply, together with modifications of my invention. A suitable overload protection, and a pushbutton of Whether the hereinbefore mentioned time dethe time-delay undervoltage type. The pushbutlay pushbutton or the two-wire master switch is ton functions in such a manner that, upon the used, after a voltage dip or temporary voltage occurrence of a severe voltage dip or interruption failure, it is desirable that the motor be reconof voltage, the magnetic contactor opens; then, nected to the supply source just as promptly as if the duration of the voltage dip or interruption the voltage on the supply source returns to a of voltage is less than the time setting of the value permitting the motor to exert its normal pushbutton mechanism, whichmay be set for, torque, or near normal torque, in order that there say, two seconds, the Voltage, upon returning may be a minimum of decrease in the speed of to the pick-up value of the contactor, will cause driven apparatus and also a minimum disturb the contactor to reclose automatically. If, howance on the power circuit when the motor is reever, the voltage fails to return to the pick-up connected to the line. In certain manufacturing value of the contactor within the time setting situations one or the other or both of these conof the pushbutton, then the pushbutton acts to siderations may be critical. open the energizing circuit for the coil of the ter an uta r v ta dip l w enough magnetic contactor. The contactor is thus preto cause the contactor, normally connecting he vented from reclosing when the voltage on the motor to the supply, to drop out, the value to: supply does return. It is then necessary to start which voltage must return before the motor will. the motor by manual operation of the pushbutbe reconnected to the line depends upon the volt- 'ton. age at which the contactor will pick up. Commer- In some cases, where no operation hazards are 40 cial contactors are regularly guaranteed to pick involved .through unexpected restarting of a moup at 85% of rated voltage. tor that has stopped by reason of a voltage fail- Many power systems feed important load cenure or voltage dip, the control switch may be of ters through two or more circuits. Upon occurthe two-wire type or "maintained-single-contact rence of a fault, the operation of protective re-- type, sometimes designated as a low voltage relays and circuit breakers controlling the supplylease. In the run position such a device pering'feeders may be such that the voltage at the manently connects the contactor coil of the motor load, following an initial low value, is restored contactor to the supply source, whereas in the to anintermediaterange before complete restoraoff position, the coil of the motor contactor is tion to normal value. For example, at a certain simply disconnected from the supply source. industrial plant location it has been reported that A broad object of my invention is to reduce to the voltage on such occasion may be, say, from a minimum the time of disconnection of electric zero to 40 percent for a half-second period, folapparatus from a source of supply by reason of a lowing which it may rise to a value in the order voltage dip, or voltage failure, of the source of of percent for an additional second, before supply. restoration to normal value. The dotted line in Fig. 2 represents this pattern of voltage behavior.

Application of 70% of full voltage to a motor allows the average motor to deliver approximately full load torque provided the motor is now below pull-out speed. In fact the torque at-any speed will be approximately half (49%) of the torque that would be delivered at full voltage. Ordinarily, commercial magnetic oontactors will not close at 70% of normal voltage but only at about 85%, or more, of normal voltage. This means, in the illustration given above, that the motor remains disconnected from the power supply for about one and a half seconds.

If the motor could be reconnected to the power line when the voltage is up to 70% of normal, the motor will have slowed down less and will thus be able to develop its full speed torque. If the reconnection of the motor to the supply is delayed for one and a half seconds instead of only one half second, its speed may by that time, be below the pull-out speed. The motor may thus stall. In any event, for some applications, the delay may be critical.

With my systems of control, the motor can be reconnected to the supply with minimum delay bycausing the motor contactor to reclose at approximately 70% of normal voltage. At the present state of the art, production of contactors to pick up at 70% of normal voltage is not a commercial expedient.

In Fig. l, I show a motor M disposed to be sharp dip in voltage to practically zero voltage occurs as indicated by the curve in Fig. 2, the contactor 4 will open contact members 8, I0 and II, or drop out, and contact members 6 close, shunting resistor I5. Since the resistor I5 is now not in the circuit of coil 8, the contactor 4 will pick up at 70% of normal voltage to connect the motor to the supply. This will occur at point D on the curve or only one half second after the voltage dip began. The moment the contactor picks up the resistor I5 is, of course,

reinserted in the coil circuit but, since it obviously :takes very much less current to hold a contactor I in than to pick it up, the contactor holds contact members 9, I0 and II closed.

The motor outage is thus reduced from one and a half seconds, down to one half second.

, value of resistor I5 and the characteristics of connected to the power leads I, 2 and 3 by the contactor 4. This may be effected as follows: By operation of the Start switch a circuit is established from lead 2 through contact, members 5 of the stop switch, backcontact members 6 of contactor 4, contact members 1 of the overload protective relay IB, coil 8 of the contactor 4 to lead I. Operation of the contactor 4 closes contact members 9, I0 and II connect-, ing the motorleads I2, I3 and I4 to leads I, 2 and 3, respectively. Operation of contactor 4 also opens the shunt circuit for resistor I5 thus placing this resistor in series with coil 8 of the contactor 4.

Instead of the usual practice ofdesigningthe contactor 4 to operate on the full circuit voltage, I insert the resistor I5 in series with coil Band utilize the auxiliary contact. members 6 of the contactor 4 to short-circuit the resistor I5 when the coil 8 is deenergized and to insert the resistor in the coil circuit when the coil is energized.

The ohmic value of the resistor I5 and the characteristics of the contactor 4 are chosen to have a definite relation. The proper relation.- ship can be effected with a contactor of ordinary commercial design characteristics excepbthat the coil is wound, or otherwise designed, for a voltage lower than the normal supply voltage.

The relationship should be such that:

(1) When resistor I5 is in series with coil 8 the combination willoperate on normal circuit voltage without overheating;

(2) When resistor I5 is in series with coil 8 the contactor 4 will hold in at any voltage down,

to 65% of normal supply voltage;.,and

(3) Without resistor I5 in series withcoil 8, that is. the, resistor I5, shunted, the contactor 4 will pick up at a voltage of 70% offlthe normal supply voltage.

Assuming that the motor isrunning, that is, the circuits are all established as hereinbefore traced, namely, contact members 5, l. 9, I0, and II are-closed, and contact, members ,6 areopen'.

If the voltage now fails very temporarily, or a contactor 4 are such that:

(1) When resistor I5 is in series with coil 8, the combination will operate at normal circuit voltage without overheating;

(2) When resistor I5 is in series with coil 8 contactor 4 will hold in at any circuit voltage above approximately 80% of normal; and

(3) Without the resistor I5 in series with coil 8 contactor 4, that is, when resistor I5 is shunted, the relay will pull in at of normal voltage.

If the motor isrunning and a voltage dip occurs relay I! will drop out and contact members 26 close. Contactor 4 also drops out. Then upon an increase of voltage to 70% of normal contactor 4 picks up reconnecting the motor to the line. Upon restoration of voltage to normal (or any voltage of of normal or above) relay II picks up to open contact members 26 placing the resistor I5 in the coil circuit. No other change occurs in the control circuit and the equipment is returned to normal operation.

In the modification shown in Fig. 4, relay I1 is merely connected to leads I2 and I3, that is, on the motor side. This arrangement prevents the possibility of the small relay II of ever operating ahead of the larger contactor 4, as might occur when the voltage is increased practically instantaneously from partial to full normal circuit voltage.

I do not wish to be limited to the exact embodiments of my invention herein disclosed inasmuch as various changes and modifications, particularly after having had the benefit of my teachings, will appear to those skilled in the art. My invention is to be limited only by the appended claims.

I claim as my invention:

1. In a system of control for, an electric device. in combination, a source of electric energy,

an electric device, an electromagnetic contactor for connecting said device to the source of energy, said contactor having an actuating coil and an energizing circuit for the coil, said energizing circuit including one terminal of the source of supply, a switch, a resistor, the said coil of the contactor and another terminal of the supply, a relay having an actuating coil so connected to the source of electric energy that opening of the contact members of said electromagnetic contactor deenergizes the relay, contact members operable by the deenergization of said relay for shunting said resistor.

2; In a system of control for an electric motor, in combination, a source of supply subject to voltage clips of relatively short duration, an electromagnetic contactor, having an energizing coil, for connecting and disconnecting the motor and the source of supply, a switch for closing the energizing circuit of the coil of the contactor to thus effect connection of the motor to the supply, an impedance normally in circuit with said coil, electromagnetic means connected on the motor side of said contactor and which electromagnetic means is thus subject to be deenergized by a predetermined voltage dip of said source, and means operable by the deenergization of said electromagnetic means for shunting said impedance.

3. In an electric system, in combination, a source of energy normally having a constant voltage but which at times is subject to temporary voltage dips, which clips of voltage under the worst conditions may involve an almost instantaneous decrease of voltage to zero, then a rise of voltage to about 70% of normal voltage in about half a second and then to normal voltage in about one and one half additional seconds or longer, a load circuit, an electromagnetic contactor of conventional commercial design but having a coil and impedance series circuit combination that will hold the contactor in continuously in closed position to interconnect the load circuit with the source of supply without overheating so long as the voltage of the supply does not drop below about of normal value, an auxiliary relay, connected to the load circuit and thus energized only when the contactor is in closed position, having the characteristic of picking up at slightly above of normal circuit voltage and a dropout voltage equivalent to that of said contactor, contact members controlled by said relay for shunting the impedance when a voltage dip occurs sufficient for the contactor to drop out to deenergize the relay whereby the coil of said contactor is connected directly to the source of supply, said coil of the contactor being so chosen that it pulls the contactor in when the voltage of the supply has risen to about 70% of normal voltage.

4, A motor control system comprising, in combination, a source of supply for said motor, electromagnetic contacting means energized by said source of supply for connecting and disconnecting said motor and said source of supply, said contacting means having the characteristic of operating to connect said motor to said source at a predetermined minimum value of potential of said source suflicient to produce normal torque of said motor, a resistor in series with said contacting means, a relay connected to the source of supply on the motor side of said contactor having contacts adapted to shunt said resistor upon deenergization thereof whereby said contacting means connects said motor to said source of supply with said resistor shunted and thereafter has said resistor inserted in series therewith by operation of said relay.

MERRITT A. HYDE. 

